The French wanted Indochina's rice, oil, iron ore, rubber ,and tin.
They claimed the overall reason for their intrusion of Indochina was to convert the Indochinese to Christianity.
The French fought two wars to acquire all of the land shown in the map above Sino-French war, and the Franco-Siamese war and controlled all of Indochina in the late 1800's
The French controlled Indochina until 1940 when some of the Indochinese fought back during WWII, causing the French-Thai war, when the French were weaker and Japan stood in at the end of the war in May 1941 giving Thailand a small land gain.
During WWII a French-Educated dissident named Ho Chi Minh was formulating plans and was supported by the U.S. but after WWII the U.S. no longer supported him after he was declared the president of North Vietnam and began the First Indochina war. Ultimately he would win the the help of communist and Vietnamese nationalists and in March 1954 Vietnam gained independence from France.
googleimages.com Ho Chi Minh